Particulate mass concentration (PM10, PM2.5), aerosol number concentration (NOPC), aerosol size distribution and atmospheric visibility were simultaneously measured in Beijing in the summer and winter of 2001. From the profuse data collected, the following conclusions can be drown: (1) there are two peaks for the average diurnal variations of PM10 and NOPC in the summer, an obvious valley can be seen between 12:30 and 15:00 for their average diurnal variation in the winter; (2) there is one single peak for the average diurnal variation of atmospheric visibility in the summer,while there are two peaks in the winter; (3) PM10 and NOPC are usually smaller in the summer than those in the winter; (4)the average visibility is much lower during 00:00～09:00 in the summer than that in the winter, because of the very high relative humidity and higher concentration of the large particles (r＞1.25 μm) in the summer during the period; (5) aerosol size distributions usually depend closely on season and relative humidity at the same place.